1. Field of the Invention
The present invention relates to a radio communication system including a plurality of radio base stations, a radio base station, a radio communication method, and a program for the radio communication system.
2. Description of Related Art
In general, in a radio communication system in which a radio section is half duplex such as a wireless network defined in the IEEE802.11 standard, when each of radio base stations transmits a frame, the radio base station needs to once check whether a medium (a communication tolerance in a communication band) is busy, i.e., whether frame transmission at a communication frequency can be done.
In such a radio communication system, when the radio base station detects presence of a transmission frame from another radio base station in a medium, the radio base station performing the transmission temporarily stops transmission processing and resumes the transmission processing after a predetermined waiting time passes.
In the Medium Access Control (MAC) in the IEEE802.11 wireless LAN, the Polling Coordination Function (PCF) and the Distributed Coordination Function (DCF) are prescribed as radio channel access methods.
In the PCF, usually, a radio base station sequentially allocates a transmission right to respective radio communication terminals. Since accesses to a radio channel are centrally controlled in this way, collision of frames does not occur in a Contention Free Period (CFP) in which the PCF works.
On the other hand, in the DCF, respective radio communication terminals acquire a transmission right for a radio channel in competition with one another. Since the respective radio communication terminals perform this transmission right acquisition competition in a distributed manner, collision of frames occurs in a Contention Period (CP) in which the DCF works.
As such a conventional radio communication system, there is a radio communication system in which, when interference occurs between two access points because the access points use communication channels of the same frequency band, an access point arbitrator gives permission for communication to one of the access points and puts the other access points on standby (see, for example, Japanese Patent Application Laid Open No. 2004-48356 (document 1)).
There is a radio communication system that uses scheduling information included in a scheduling information parameter set to decide time during which an access point functions as a point coordinator (see, for example, Japanese Patent Laid Open Publication No. 2004-187297 (document 2)).
However, the waiting time in the conventional radio communication system increases exponentially according to the number of times the medium is found busy at a point of transmission. Thus, even when a transmission frame itself is short, time required for transmission of one frame increases because of the waiting time, causing deterioration in transmission frame efficiency.
When the time required for transmission of one frame increases in this way, in the case of data communication, transmission speed of data falls and, in the case of voice communication, the number of simultaneous calls is decreased.
When it is attempted to increase the number of radio base stations or replace the radio base stations with a high-performance model, in which a maximum number of simultaneous connections is large, in order to reduce the waiting time, cost rises.
In documents 1 and 2, radio base stations or system control devices prevent mutual interference among access points.
As described above, when the CFP in which the radio communication system performs allocation control for a transmission right and the CP in which the radio communication system does not perform the allocation control are defined, documents 1 and 2 do not take into account a period for control for sequentially allocating a transmission right from the radio communication system to the respective radio communication terminals.